The discovery that endogenously produced H2S is a signaling molecule that mediates varied physiological effects in mammals ranging from neuromodulatory to cardioprotective, has spurred enormous recent interest in understanding its biology and in exploiting its pharmacological potential. Enzymes involved in sulfur metabolism catalyze the biogenesis of H2S and include: cystathionine -synthase (CBS), - cystathionase (CSE) and mercaptopyruvate sulfurtransferase (MST). However, despite the rising interest in H2S biochemistry, fundamental questions remain regarding regulation of its production, its mechanism of action and its destruction. In this study, we propose to address significant gaps in our understanding of the reaction mechanisms and regulation of H2S production by addressing the following specific aims. (i) Structure- function studies on H2S biogenesis will focus on elucidating the reaction mechanisms of CBS, CSE and MST and conducting a high throughput screen using a small molecule library as a platform for in vitro and ex vivo screening of activators and/or inhibitors of CBS and CSE. (ii) Regulation of CBS, CSE and MST by redox, androgens and sumoylation will be investigated to determine how these parameters modulate H2S production, affect transsulfuration flux and localization of glutathione pools during the cell cycle. The impact of the proposed studies is both fundamental (i.e., gaining insights into the operation and regulation of H2S-generating enzymes) and translational (i.e., informing therapeutic options for circumventing metabolic blockades, screening for activators/inhibitors of H2S production and characterizing testosterone regulation of the transsulfuration pathway in androgen-responsive prostate cancer).